1. Field of the Invention
The present invention relates to an optical element with a microprism structure, in particular for use as a cover for a luminaire and to a method for producing the same.
As a result of using such optical elements or luminaire-covers, the angle of emergence of light beams from a luminaire, for example, is to be limited in order to avoid or at least diminish any dazzlement for the viewer. In addition, of course, such an element also provides mechanical protection for the luminaire and in particular for the light source in the interior of the luminaire.
2. Description of the Related Art
An optical element of the kind mentioned by way of introduction is known, for example, from Austrian Patent AT-B-403,403. The known optical element, which is used as a luminaire-cover, has, on its side facing the lamp, pyramidal profiled portions that are arranged in rows and lines, so-called microprisms, which are formed as truncated pyramids and have an upper boundary face (light-entry face) that lies parallel to the base (light-emergence face). The whole luminaire-cover is made totally of a crystal-clear or transparent material.
A truncated pyramid or microprism according to AT-B-403,403 is shown in FIG. 1 for the purpose of explaining the problem which the present invention seeks to solve. As described in greater detail in the patent specification, the known microprism 1 has a top surface 2 which serves as a light-entry face, a root 3 which is arranged so as to be parallel to the latter and serves as a light-emergence face, and obliquely extending side walls 4 in such a way that the microprism 1 assumes the shape of a truncated pyramid. The angle of emergence xcex3 of the emerging light beams in relation to the perpendicular to the light-emergence face 3 is to amount to at most approximately xcex3max≈60-70xc2x0, preferably xcex3max≈60xc2x0, in order to avoid any dazzlement for the viewer when looking at the luminaire from the side. In order to achieve the highest possible degree of optical efficiency at the same time, an optimum ratio of the dimensions of the truncated pyramid d:h that is dependent upon the refractive index n of the material of the truncated pyramid emerges for the microprism 1 made of transparent material. Furthermore, an optimum furrow angle xcex4 between adjacent microprisms 1 of approximately 8-9xc2x0 also emerges with a grid dimension of the microprisms of approximately 700 xcexcm.
Given the parameters mentioned above, even for those light beams that strike the edges 5 directly or strike immediately next to the edges 5 of the truncated pyramid 1 and just pass by the edges 6 between the truncated pyramids 1, the result is that they emerge from the plane of the light-emergence face 3 at an angle of emergence of xcex3 less than 60xc2x0.
It has, however, been found in practice that in terms of manufacturing techniques it is extremely difficult or almost impossible to observe a furrow angle xcex4 of approximately 8-9xc2x0. Currently, furrow angles xcex4 of approximately 15xc2x0 can be realized with a sufficiently high level of precision and reproducibility. Whilst even with a realistic furrow angle xcex4≈15xc2x0, an angle of emergence of xcex3max≈60xc2x0 can be achieved, given a constant height h of the truncated pyramid 1 the consequence of this though is that on account of the thus diminished area of the light-entry face 2 the degree of optical efficiency of the luminaire-cover is reduced in a corresponding manner from approximately 75-80% to approximately 65%. Alternatively, in order to maintain the extent of the light-entry face 2 the height h of the truncated pyramid 1 could also be diminished. In this case, however, the maximum light-emergence angle xcex3max≈60xc2x0 would no longer be observed, since light beams, which enter the microprism 1 directly at the edges 5 of the light-entry faces 2 and just pass by the edges 6 between adjacent truncated pyramids 1, leave the luminaire-cover at a flatter angle.
A further luminaire-cover or a further optical element of the kind mentioned by way of introduction is known, for example, from WO 97/36131. The microprism structure disclosed in this printed specification has, on the one hand, on the side walls of the truncated pyramids a reflective cover in order to prevent light from emerging from the side walls of the microprisms and reducing the degree of efficiency of the luminaire-arrangement and, on the other hand, on the side of the light-emergence faces of the microprisms a lens system in order to concentrate the light beams in a direction substantially perpendicularly in relation to the plane of the light-emergence face. However, the structure of this luminaire-cover is comparatively complicated and therefore more expensive in terms of manufacturing techniques in comparison with the luminaire-cover known from AT-B-403,403.
Basing considerations on the afore-mentioned prior art, it is an object of the present invention to provide an optical element of the kind mentioned by way of introduction that avoids the disadvantages described above in the case of the prior art and which in particular with a high degree of efficiency in terms of lighting techniques guarantees an angle of emergence of the light beams from the optical element with which dazzlement of the viewer is avoided.
This object is achieved by means of an optical element according to the present invention.
Owing to the fact that the light-entry face of the optical element formed by the top surfaces of the microprisms are formed convexly or concavely in a continuous or non-continuous manner, the light beams coming from a lamp strike the light-entry face of the element at a different angle of incidence than in the case of a light-entry face that is aligned so as to be parallel to the light-emergence face. As a result of suitable selection of the curvature or degree of curvature which besides also depends upon the material of the element core and in particular upon its refractive index, the light beams can only emerge from the light-emergence face at an angle of emergence of at most approximately 60xc2x0 in relation to the perpendicular of the light-emergence face without the degree of efficiency of the optical element in terms of lighting techniques being diminished thereby. A concave formation of the top surfaces of the microprisms causes the light beams, which impinge upon the top surfaces, to be refracted into the microprism structure at steeper angles and not to leave the optical element at an angle that is too flat. A convex formation of the top surfaces of the microprisms, on the other hand, causes the light beams, which impinge upon the top surfaces at a flat angle, to be refracted into the microprism structure at flatter angles, to be totally reflected therefore at the opposing structure flank and to leave the optical element at a sufficiently small angle of emergence.
The formation of the top surfaces of the microprisms is preferably realized by means of a convex or concave curvature or step formation, with the curvature or step formation not necessarily having to extend over all of the top surfaces.
The plate-like core of the optical element in accordance with the present invention is either mechanically worked out of a transparent block or produced by pouring or injecting the transparent material into an appropriate mould and subsequently applying pressure thereto.
Further advantageous configurations and further developments of the present invention are shown and described herein.